1. Field of the Invention
The present invention relates to an optical communication system and an optical reception apparatus using a synchronous polarization scrambler for scrambling the polarized state of a signal light according to a polarization modulation signal of a repetition frequency which coincides with the bit rate of the signal light.
2. Description of the Related Art
Heretofore, in optical transmission systems for long distances of over several thousands of kilometers and across the ocean, the optical signal has been converted to an electric signal, and transmitted using an optical regenerative repeater for retiming, reshaping and regenerating. Recently, however, optical amplifiers have been put to practical use, and an optical amplifier-repeated transmission method which uses an optical amplifier as a linear repeater has been studied. By substituting an optical regenerative repeater for an optical amplifying repeater, the number of parts in the repeater is greatly reduced, and the reliability is secured while the manufacturing cost can be greatly reduced.
In 1993, M. G. Taylor pointed out a phenomenon where noise light generated by a light repeating amplifier depends on the polarized state of a signal light, and the excessive noise light increases (polarization hole burning). Since due to the polarization hole burning, the mean value of the signal light to noise light ratio (optical SNR) decreases while the fluctuation of the optical SNR increases, this becomes a big problem in performing optical amplifier-repeated transmission.
As a measure against this, for example, polarization scrambling which can positively vary the polarized state of a signal light on the transmitting side has been proposed. FIG. 21 shows a schematic construction of a conventional optical communication system using polarization scrambler.
The conventional system shown in FIG. 21 comprises an optical transmitter 100 for sending a polarization-scrambled signal light to a transmission system 200, and an optical receiver 300 for receiving the signal light from the transmission system 200 and performing decision processing and the like. With the optical transmitter 100, a signal light having a predetermined bit rate which has been modulated according to the transmission data is projected from a signal light generator (E/O) 101, and the signal light is polarization-scrambled by a polarization scrambler (PS) 102 according to the polarization modulation signal and output to the transmission system 200. As a method for the polarization scrambling, there are, for example, a method using a phase modulator, a method for imparting a stress from a side of an optical fiber, and a method using two light sources. In the optical receiver 300, the signal light from the transmission system 200 is converted into an electric signal by a light receiving device (O/E) 301, and is subjected to the decision processing with a decision circuit (DEC) 302.
For example, in an experiment with a bit rate of 5.33 Gb/s and a transmission distance of 8100 km, in 1994, F. Heismann et al. attained a Q value improvement by 4 dB in 40 kHz and by 5 dB in 10.66 GHz for the repetition frequency in the polarized state, with a polarization scrambler using a phase modulator of lithium niobate (LiNbO3) with a designated input polarized state of 45 degrees. The former repetition frequency is lower than the bit rate, and is referred to as low-speed polarization scrambling, while the latter is equal to or higher than the bit rate, and is therefore referred to as high-speed polarization scrambling. High-speed polarization scrambling has the effect of suppressing fluctuations in the optical SNR due to the polarization dependence loss of the optical transmission path and the optical amplifying repeater, and hence the improved amount thereof is large.
A polarization scrambler using a phase modulator will now be briefly described. The phase difference xcex94Ø(t) of light in the TM mode and the TE mode generated by the phase modulator can be expressed by the following expression:
xcex94Ø(t)=xcfx80/xcex[(ne3xcex333xe2x88x92no3xcex313)V(t)Lxcex93]
where, xcex denotes the optical wavelength, xcex333 and xcex313 denote the electrooptical constant of the TM mode and the TE mode, ne and no denote the optical refractive index of the TM mode and the TE mode, V(t) denotes the applied voltage, L denotes the length of an electrode, and xcex93 denotes a reduction coefficient of the applied voltage. By using this phase difference xcex94Ø(t) of light in the TM mode and the TE mode, the polarization scrambler using the phase modulator can change the polarized state of the signal light.
Furthermore, as one method for realizing a large capacity of the optical transmission system, a wavelength division-multiplexing (WDM) optical transmission system is noted, which multiplexes and transmits light signals having two or more different wavelengths in a single optical transmission path.
With the WDM optical amplifier-repeated transmission method combining the WDM optical transmission method and the above-mentioned optical amplifier-repeated transmission method, it is possible to amplify light signals having two or more different wavelengths in the block using an optical amplifier, hence a large-capacity and long-distance transmission can be realized with a simple (economical) construction.
With the WDM optical amplifier-repeated transmission method, it is important to reduce the deterioration of the transmission characteristics due to the nonlinear effects of the optical transmission path. The incidence efficiency of, for example, the four-wave mixing (hereinafter referred to as xe2x80x9cFWMxe2x80x9d) which is one of the nonlinear effects becomes maximum, when the polarized state of some signal lights coincide. Therefore, since it can be so set that the coincidence of the polarized state of some signal lights cannot be positively maintained by, for example, carrying out high-speed polarization scrambling, it is possible to reduce the incidence of the four-wave mixing.
As one example, the present inventors confirmed in 1996 in an experiment with four-wave multiplexing, a bit rate of 5.33 Gb/s, and a transmission distance of 4800 km, that by performing high-speed polarization scrambling in which the repetition frequency in the polarized state was twice as high as the bit rate, the incidence of the four-wave mixing was reduced thus improving the transmission characteristics. Furthermore, the WDM transmission method in which polarization scrambling is performed is described in detail in Japanese Unexamined Patent Publication No. 9-149006 which is the prior application of the present applicant.
Moreover, as one of the other important problems with the WDM optical amplifier-repeated transmission method, there can be mentioned the reduction of the channel interval, that is, the increase in the number of wavelength multiplexing. However since the signal light subjected to the high-speed polarization scrambling has an expanded spectrum, this becomes a hindrance when the high-density wavelength multiplexing is realized.
Therefore, N. S. Bergano et al. proposed in 1995 polarization scrambling in which the spectrum expansion in the signal light is relatively small and the repetition frequency is the same as the bit rate. Such polarization scrambling is referred to herein as synchronous polarization scrambling. In this proposal however, there is required a construction such that the intensity modulation and the polarization scrambling of a signal are synchronized in order to improve the transmission characteristics.
As the optical transmitter for performing the synchronous polarization scrambling, for example, there is a transmitter, as shown in FIG. 22, comprising a light source (LD) 101A for generating continuous light, an intensity modulator (IM) 101B for modulating the intensity of the light from the light source 101A, a first driving circuit (DRV) 101C for driving the intensity modulator 101B, a wave-shaping circuit 101D for synchronizing an input signal IN with an oscillation signal and sending the input signal to the first driving circuit 101C, a polarization scrambler (PS) 102A for polarization scrambling the signal light from the intensity modulator 101B, a second driving circuit (DRV) 102B for driving the polarization scrambler 102A, and a delay circuit 102C for sending the polarization-modulated signal in which the oscillation signal is delayed to the second driving circuit 102B.
The technology relating to the polarization scrambling as described above can be roughly divided into three according to the repetition frequency: a low-speed polarization scrambling, a synchronous polarization scrambling and a high-speed polarization scrambling. Here, the results of comparing and studying the characteristics when each polarization scrambling is performed, and when the polarization scrambling is not performed are shown in Table 1 below.
In Table 1, Br denotes oscillation frequency used for the bit rate, PDG denotes polarization dependence gain, and PDL denotes polarization dependence loss.
One of the important requirement items for putting a large-capacity and long-distance transmission system into practical use is a reduction of the number of optical amplifying repeaters by extending the interval between repeaters. For that purpose, it is necessary to reduce the incidence efficiency of FWM and to realize a high-output optical amplifying repeater. As a method to reduce the incidence efficiency of FWM, synchronous polarization scrambling or high-speed polarization scrambling is effective, as shown in Table 1. Furthermore, with synchronous polarization scrambling or high-speed polarization scrambling, though not an items of Table 1, it is possible to make the output higher by about 3 dB than with the low-speed polarization scrambling.
High-speed polarization scrambling is especially effective when the bit rate is about 2.5 Gb/s. When the bit rate becomes about 5 Gb/s or higher, however, the expansion of the spectrum in the signal light cannot be ignored, hence the wavelength dispersion tolerance becomes small. Moreover, it is necessary to have a wide channel interval. On the other hand, with synchronous polarization scrambling, since the expansion of the spectrum in the signal light is about half of that with high-speed polarization scrambling, the requirements of the wavelength dispersion tolerance and the channel interval can be alleviated.
Therefore, with high-speed optical communication systems having a bit rate of 5 Gb/s or higher, synchronous polarization scrambling is more effective.
With high-speed optical communication systems using synchronous polarization scrambling, however, when the signal light polarization-modulated by using the modulation signal in the repetition frequency which coincides with the bit rate (hereinafter, referred to as xe2x80x9cpolarization CLK frequency) is transmitted through the transmission path, an intensity modulation based on the Kerr effect due to the phase modulation component resulting from the polarization modulation thereof occurs, and the polarization CLK frequency is superimposed on the light emitting side of the waveform after the transmission. Furthermore, due to polarization dependence loss (PDL) of the optical transmission path, the optical amplifying repeater or the like, there is the possibility that the polarization modulation component is converted to intensity noise thus increasing the Q value fluctuation.
The results of transmission experiments performed under the condition of, for example, a bit rate of 5.3 Gb/s, eight wavelength multiplexing, and a transmission distance of 2679 km are shown in FIG. 23xcx9cFIG. 25. FIGS. 23(a) and (b) show the reshaped waveform and the electric spectrum of the signal light transmitted from the optical transmitter (signal light before transmission subjected to synchronous polarization scrambling), and FIGS. 24(a) and (b) show the reshaped waveform and the electric spectrum of the signal light received by the optical receiver (signal light after transmission). In addition, FIGS. 25(a) and (b) show the reshaped waveform and the electric spectrum after the received signal is passed through an equalizing filter.
First with regard to the change before and after transmission, the reshaped waveform before transmission in FIG. 23(a) is approximately vertically symmetrical, but it is seen that the reshaped waveform after transmission shown in FIG. 24(a) is not vertically symmetrical, because the polarization CLK frequency component is superimposed on the light emitting side (upper side in the figure). Moreover, with regard to each electric spectrum in FIG. 23(b) and FIG. 24(b), it is seen that the power of the polarization CLK frequency fo component before and after the transmission increases to about 16 dB (about forty times).
The change in the signal light before and after transmission can be considered to results from the Kerr effect due to the phase modulation as described above, and results from the polarization dependence loss. With regard to the former case, since the correlation of the phase modulation and the intensity modulation is strong, if the state of the transmission data is reconciled with the polarized state (phase) modulation, it is possible to obtain a waveform having a large opening portion as shown in FIG. 24(a). With regard to the latter case, however, since the change in the signal light occurs depending upon the state of the optical transmission path, it is difficult to control the occurrence.
Furthermore, the spectral component in the vicinity of the polarization CLK frequency fo is within a range of the polarization CLK frequency fo xc2x150 Hz, since the polarization CLK frequency fo is slightly modulated due to the polarization fluctuation (the major frequency component is about 50 Hz or below) in the optical transmission path. Accordingly, the peak of the polarization CLK frequency fo in FIG. 24(b) has a range of about 100 Hz. In addition, the peak on the right hand side in FIG. 23(b) and FIG. 24(b) is a harmonic component twice as high as the polarization CLK frequency.
As described above, when the synchronously polarization-scrambled signal light is transmitted, the signal light affected by the polarization modulation is received by the optical receiver. The optical receiver then converts the received signal light to an electric signal, and extracts only a necessary frequency component through the equalizing filter to perform decision processing and the like. In general, as the equalizing filter used for the optical receiver, a Bessel-type filter or the like excellent in the group delay characteristic is used, and the cutoff frequency is set to be around 0.6 to 0.8 times as high as the bit rate, when an NRZ code or the like is used. With such an equalizing filter, when the synchronous polarization scrambling is performed, the frequency component of the bit rate, that is, the polarization CLK frequency fo component cannot be sufficiently cut off.
In the above-mentioned transmission experiment, when, for example, an equalizing filter having a cutoff frequency of 4.0 GHz is used with respect to the bit rate of 5.3 Gb/s, as shown in FIGS. 25(a) and (b), the reshaped waveform remains vertically asymmetric, and it is seen that the amount of attenuation of the polarization CLK frequency fo component is only about 6 dB compared to that for before passing through the equalizing filter.
When such a signal is sent to the decision circuit, there is a possibility that the reception characteristic is deteriorated due to the influence of the polarization CLK frequency fo component, which is a problem.
In view of the above problems, it is an object of the present invention to provide an optical communication system and an optical reception apparatus which can reduce the noise component generated when a synchronously polarization-scrambled signal light is transmitted.
The technique for suppressing an unnecessary component for the low-speed polarization scrambling has been proposed in Japanese Unexamined Patent Publication No. 9-8742 being the prior application of the present applicant. However the present invention is intended for synchronous polarization scrambling and has a different construction.
Therefore, the optical reception apparatus of the present invention is an optical reception apparatus for receiving and processing a synchronously polarization-scrambled signal light via an optical transmission device, using a synchronous polarization scrambler which scrambles the polarized state of the signal light in accordance with a polarization modulation signal of a repetition frequency which coincides with the bit rate, and comprises a noise reducing section for reducing only a noise component generated based on the synchronous polarization scrambling performed with respect to at least one of, the signal light before photoelectric conversion and an electric signal after photoelectric conversion.
With such a construction, when the synchronously polarization-scrambled signal light is transmitted to the optical reception apparatus via the optical transmission device, noise resulting from the polarization modulation is generated because of the transmission of the synchronously polarization-scrambled signal light through the optical transmission device. The optical reception apparatus receives the signal light containing such a noise component, but only the above-mentioned noise component is reduced by sending the received signal light or the photoelectrically converted electric signal to the noise reducing section. Thus, by performing the reception processing using a signal in which the noise is reduced, excellent reception characteristics can be obtained.
As a specific example of a noise reducing section for reducing the noise component contained in the received signal, in the electric signal stage, a band reject filter having a rejection bandwidth centered on the repetition frequency may be used. With this band reject filter, it is desirable that the amount of attenuation of the noise component in the rejection bandwidth be not less than 3 dB, and it is preferable that the rejection bandwidth be not less than 100 Hz. Moreover, the group delay amount in a range of from DC to 0.8 times the frequency fo is desirably not higher than 10% of the period for one bit given from the bit rate.
Furthermore as another specific example of a noise reducing section for reducing the noise component, in the electric signal stage, a low-pass filter which intercepts noise components equal to or higher than the repetition frequency may be used. With the low-pass filter, it is desirable that the amount of attenuation of noise components equal to or higher than the repetition frequency be not less than 3 dB, and it is preferable that the group delay amount in a range of from DC to 0.8 times the frequency fo be not higher than 10% of the period for one bit given from the bit rate.
On the other hand, as a specific example of a noise reducing section in which the noise component contained in the received signal is reduced in the light signal stage, a light reducing filter having a narrow bandwidth capable of intercepting the noise component affected by the repetition frequency may be used. It is desirable that this light reducing filter have a bandwidth of which center frequency corresponds to the frequency of the signal light and which is narrower than twice the width of the repetition frequency.
By constituting the noise reducing section as described above, it is possible to reliably reduce the noise component contained in the received signal light or the electric signal to which the signal light is photoelectrically converted. Naturally, a construction for reducing the noise component for both the signal light and the photoelectrically converted electric signal is also possible.
Furthermore, the noise reducing section using the light reducing filter may include a light reducing filter control section which directs the center frequency in the bandwidth of the light reducing filter to follow the frequency of the signal light.
By providing the light reducing filter control section, then even if the frequency of the signal light fluctuates, the center frequency of the bandwidth of the light reducing filter follows the fluctuation, and hence even a light reducing filter having a narrow bandwidth can reliably intercept the noise component.
The light receiving apparatus described above may include a signal extraction section for extracting a signal of the repetition frequency component, in the stage prior to the noise reducing section, to output the signal extracted by the signal extraction section as a monitoring signal for monitoring the polarization dependence of the optical transmission device. As the signal extraction section, a circulator for extracting the noise component intercepted and reflected by the noise reducing section may be used. In addition, the signal extraction section may include a branching section for branching a part of the input signal to be sent to the noise reducing section, and a band-pass filter for passing only the component of the repetition frequency and the component in the vicinity thereof among the signals branched by the branching section.
With such a construction, the signal of the repetition frequency component contained in the received signal is extracted by the signal extraction section. The signal of the repetition frequency component is a signal reflecting the polarization dependence of the light transmitting device. Accordingly, it becomes possible to monitor the polarization dependence of the optical transmission device by designating the extracted signal as the monitoring signal, and by analyzing the monitoring signal, for example, in the time domain or in the frequency domain.
Furthermore, the optical reception apparatus including the signal extraction section may include a clock generating section for generating a clock signal for the reception processing, using the signal extracted by the signal extraction section.
The signal extracted by the signal extraction section has a repetition frequency which coincides with the bit rate, hence it can be used as the clock signal used for the reception processing. As described above, if the clock generating section is provided to generate the clock signal from the extracted signal, the clock signal for the reception processing which has heretofore been obtained by retiming the data signal can be easily obtained.
The optical communication system using the synchronous polarization scrambler of the present invention is an optical communication system which comprises a light transmitting device, including a synchronous polarization scrambler, for scrambling the polarized state of a signal light according to the polarization modulation signal of the repetition frequency which coincides with the bit rate for transmitting the synchronously polarization-scrambled signal light to an optical transmission device, and a light receiving device for receiving and processing the signal light transmitted from the light transmitting device via the optical transmission device, wherein the light receiving device includes a noise reducing section for reducing only the noise component generated based on the synchronous polarization scrambling performed with respect to at least one of the signal light before the photoelectric conversion and the electric signal after the photoelectric conversion.
With such a construction, the synchronously polarization-scrambled signal light is transmitted from the light transmitting device to the light receiving device via the optical transmission device. At this time, since the synchronously polarization-scrambled signal light is transmitted via the optical transmission device, noise resulting from the polarization modulation is generated. The light receiving device receives the signal light containing such a noise component, but only the above-mentioned noise component is reduced by sending the received signal light or the photoelectrically converted electric signal to the noise reducing section. Thus, by performing the reception processing using a signal in which the noise is reduced, excellent reception characteristics can be obtained.
Furthermore, with the above-mentioned optical communication system the construction may be such that the light transmitting device includes a monitoring control signal superposing section which superimposes a monitoring control signal showing the state of the optical transmission device on the polarization modulation signal, and the light receiving device includes; a signal extraction section for extracting a signal of the repetition frequency component in the stage prior to the noise reducing section, and a monitoring control signal demodulating section for demodulating the monitoring control signal based on the signal extracted by the signal extraction section.
With such a construction, the signal light synchronously polarization-scrambled by the polarization modulation signal on which the monitoring control signal is superimposed is transmitted via the optical transmission device to the light receiving device, and the signal of the repetition frequency component is taken out by the signal extraction section in the light receiving device. The signal of the repetition frequency component becomes a signal on which the monitoring control signal is superimposed as well as the polarization modulation signal. Hence by demodulating the monitoring control signal with the monitoring control signal demodulating section, the transmission of the monitoring control signal becomes possible between the light transmitting device and the light receiving device.
Other objects, characteristics and advantages of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the appended drawings.